1. Field of the Invention
This invention relates to a method of electrochemical analysis and more particularly to a method of determining the unknown endpoint or endpoints in a potentiometric titration process.
2. Description of the Prior Art
Electrochemical analysis techniques involve the use of electrical properties to correlate with a particular chemical composition. One such electrochemical analysis technique is the field of potentiometry which relates to the use of the measured potential of a sample, as a cell, to determine the concentration of various ionic species within a particular sample. Potentiometry has found wide and accepted use in the titration of ionic species by monitoring the potential of the sample during titration. Potentiometric titration thus provides a relatively accurate method of determining endpoints in the characterization of samples containing ionic species. Numerous methods have been used to accomplish potentiometric titrations with the more recent being by fully automated methods providing an automatic titration.
The application of potentiometric titration must be separated into two distinct areas which present separate problems. The first involves the titration of samples having a known endpoint; the second involves the titration of samples having unknown endpoints.
In any titration the chemical characteristics of the sample must be considered, hence requiring procedural changes. These characteristics include the mutual reactivities of the titrant and the compounds within the sample, and the non-ionic or non-titratable constituent of the sample which affect the chemical interactions of the titrant and reactive compounds. The individual sample characteristics in all instances require the procedural changes which classically have been adjusted based on the experience of those analytical chemists skilled in potentiometric titrations.
For example, in the titration of samples having a known endpoint, the beginning of the titration involves the rapid addition of titrant to the sample, and upon approaching the endpoint, the titrant flow is reduced to allow the sample and titrant to react, thus taking into account any lag in equilibrium of the titrant-sample reaction.
In the potentiometric titration of samples having unknown endpoints several approaches have been used. A series of samples may be titrated to establish the desired titration rate to obtain an accurate endpoint, and once the desired rate is obtained, the final sample is used to determine the endpoint. Another method is a titration which is conducted at a very slow constant rate so that chemical equilibrium can be established with concomitant accuracy in endpoint determination.
In a conventional known endpoint potentiometric process, the titrant delivery rate, through an automatically driven burette, is controlled through a pre-programmed electrical circuit, such as is shown in U.S. Pat. No. 3,157,471, which reduces the rate of titrant delivery as the endpoint is approached. However, the endpoint is a known one with the only variable being the amount of titrant needed to reach this endpoint.
In a typical potentiometric titration the automatically driven burette delivers titrant to the sample. The potential E of the sample is graphically represented on a recorder which plots the potential as ordinate with the total volume added V, as abscissa. The plot resembles an "S" curve with the points of maximum curve inflection indicating the endpoint generally. To obtain the actual endpoint, the midpoint between the inflection points is determined which is also where dE/dt is maximum. In order to more accurately determine the endpoint, the first derivative of the potential with respect to the time, dE/dt, is plotted, or otherwise recorded, with respect to volume which gives a sharp peak at the endpoint. In this instance the rate of titrant, dV/dt, is held constant in the vicinity of the endpoint. Exemplary of such a method is U.S. Pat. No. 3,769,178.
In the automatic titration of samples having an unknown endpoint, it has been the practice to measure the potential E or its first derivative dE/dt to determine the equivalence which is indicative of an endpoint or endpoints. Measurement of dE/dt requires that the rate of titrant delivery be constant.
This method for unknown endpoint determination may not be satisfactory since the rate of delivery may be too fast to allow accurate readings due to an incapability of the titrant and sample to reach chemical equilibrium in the prescribed time, or may be too slow requiring the time of titration to be too long for efficient analysis.
As can be gleaned from these techniques, the endpoint determination is procedure dependent, rather than chemically dependent on the reaction of the titrant and the sample to achieve an accurate endpoint.
From the foregoing description, the values correlated to obtain an endpoint in the potentiometric titration process are the potential of the cell E and the volume of the titrant to endpoint V. Measuring variations of these values with respect to time and each other are valuable considerations in obtaining precise endpoints. However, as has been demonstrated, when dV/dt is a pre-determined and regulated value, the titration process is somewhat inaccurate or slow when titrating for unknown endpoints.
Thus, in accordance with the present invention, a method of potentiometric titration is provided which is dependent upon the chemistry of the particular sample and titrant.
Further, in accordance with the present invention, an automatic titrator is provided which is adapted to determine endpoints upon the chemistry of the particular sample and titrant.
Further, in accordance with the present invention, the method of titration allows for rapid and accurate endpoint determination.